Particle separation and classification are well explored need of the chemical, pharmaceutical, mineral and food industries. While particle classification in industrial processes may be required to improve the quality of a certain product, particle separation may be necessary to purify a fluid stream or to avoid problems to process equipment.
Sometimes particles are intentionally present in the process stream. This is for example the case of combustion processes based on pulverised fuels or production of pharmaceutical or specialty chemicals using powder technology. In other cases the presence of particles is unintentional. This is for example the case of some refineries streams, effluents from fluidized beds, product streams from Fischer Tröpsch reactors. Particles may have various origins: they may be part of the original feedstock and other reactant streams or they may be generated in and collected from process equipment, for example as erosion products. Particles may be solid or liquid, may have organic nature, like char, coke and gums, or inorganic nature, like salts, debris or corrosion and erosion as iron components, or debris of catalyst particles. They may be liquid, as some aqueous mists, and containing living impurities as bacteria. Shape and size may also vary greatly—from sphere to flakes, from millimeters to a few microns or less. If the particles are unwanted in the downstream process, often, a filter, or other suitable particle separation technology known in the art, removes large part of these particles prior to sensitive equipment. However, in certain processes, the problem may appear or become more severe over time, for example when erosion and corrosion are involved. Sometimes, installing a particle removing equipment as an independent unit operation prior to sensitive equipment is not possible in practice.
One specific example of problems generated by particles may be seen in naphtha hydrotreating. The feed to a hydrotreating reactor is sometimes laden with particles. When the particle laden feed is introduced into the hydrotreating reactor the particles tend to scale rapidly on the grading or the catalyst.
Reactors may thus require frequent skimming of the affected layers of the bed to contain the pressure drop build-up in the reactor. A frequency of once every 5-6 months or even of once every 2-3 months for skimming is not uncommon.
A characterization of the particles affecting a naphtha hydrotreater is seldom available. In fact the particles depend upon the naphtha feedstock or process related issues (rust, salts, gums, etc.). On-stream collection of the particles is typically not available. Thus, particle characterization relies on post-mortem analyses, which are affected by large uncertainties due to particle agglomeration and oxidation.
Similarly, process gas resulting from the regeneration of FCC (fluid catalytic cracking) catalyst is often laden with catalyst particles and catalyst debris. Such gas may be conveyed to a sulphur recovery unit, most commonly a Claus plant, for the recovery as elemental sulphur, or a WSA plant, for the recovery of sulphur as concentrated sulphuric acid. These are catalytic fixed bed reactors which are prone to plugging if exposed to a particle-laden feedstock. The particles commonly present at the exit of the FCC regenerator is generally in the 2 to 20 micron size range or below.
US2009177023 discloses a filtration tray for a fixed bed reactor with a co-current down-flow of gas and liquid.
The device can trap plugging particles contained in the liquid feed supplying a reactor functioning in gas and liquid co-current down-flow mode using a specific distributor tray comprising a filtration medium. The device is of particular application to the selective hydrogenation of feeds containing acetylenic and dienic compounds.
EP0358923 discloses a process and an apparatus for purifying a raw gas originating from the gasification of solids. In a process and apparatus for purifying raw gas from solids gasification, containing granular and dusty solids particles, a solution is to be found, by means of which solids particles of any size are largely removed from the raw gas before entry to downstream cooling devices. This is achieved when the raw gas is passed in a first purification stage from the gasification zone in a straight line in the direction of a gas-holding space, whereby the granular solids particles are precipitated at the bottom of the gas-holding space and then, in a second purification stage, the partially purified raw gas is laterally deflected from the gas-holding space and undergoes a change to a velocity reduced by a factor of at least 3 and, after a further gas deflection, is passed substantially in the vertical direction through a solids filter, where the dusty solids particles are removed from the raw gas.
In spite of the above mentioned known art, a need exists for a reactor with a particle separator to ensure prolonged effective operation of the reactor despite any particle impurities in the inlet fluid stream to the reactor.